1. Field of the Invention
The present invention relates to a propulsion motor control apparatus for a battery vehicle, and in particular to a propulsion motor control apparatus for a battery vehicle, ideally suited for use in a battery vehicle such as a battery forklift, which can realize optimum turning performance by improving feel during turning and turning efficiency even at low speeds.
2. Description of the Related Art
FIG. 5 shows an example of a motor rotation speed/current command value function for a conventional propulsion motor control apparatus for a battery forklift, and FIG. 6 shows another example of a motor rotation speed/current command value function for a conventional propulsion motor control apparatus for a battery forklift.
Here the battery forklift is a vehicle which is propelled with a battery as the power source. With this battery forklift, the two front wheels have a common axis, and vehicle propulsion is controlled by individually controlling propulsion motors provided for each front wheel.
As a means for adjusting the propulsion performance (propulsion feeling) of such a battery forklift, then as shown in FIG. 5, there is a device which effects adjustment, by increasing or decreasing a motor current limit value in the low speed region, or by changing the rising slope of the motor current value in the low speed region. Moreover, as another adjustment means, then as shown in FIG. 6, there is a device which makes the voltage applied to the motor in the middle speed region to high speed region constant by controlling a duty ratio of a PWM voltage with a battery voltage at 100%, to give for example 80% or 60% of the battery voltage.
However, with the conventional propulsion motor control apparatus for a battery forklift as described above, various problems arise. For example, if the battery consumption, which is an important element of a battery forklift, is to be reduced, then if as shown in FIG. 5, the current in the low speed region is reduced, then the uphill power will be insufficient. Moreover, if as shown in FIG. 6, the fixed duty applied to the motor in the mid speed and high speed region is reduced, then the optimum speed will not be output.
Therefore, the present inventor has proposed a control apparatus for a motor with a battery power source, which can obtain a sufficient output torque in the low speed region and which can at the same time reduce the battery consumption as much as possible, while ensuring a predetermined optimum speed (refer to Japanese Patent Application, First Publication, No. Hei 10-94114).
FIG. 7 is a schematic diagram illustrating a three wheel battery forklift with two propulsion motors to which this control apparatus for a motor with a battery power source is applied. In FIG. 7, numeral 1 denotes a battery, 2 an accelerator, 3 a propulsion motor control apparatus, 4 a propulsion motor, 5 a transmission unit, 6 a rotation sensor for detecting the rotation speed of the propulsion motor 4, and 7 a front wheel of the forklift. Here for convenience sake, only one front wheel 7 is shown. The other front wheel which is driven independently of this front wheel 7, is omitted from the figure.
FIG. 8 is a diagram showing an example of a motor rotation speed/current command value function for when an accelerator division in the battery forklift is at a maximum. This function is determined as follows.
(1) Point (a) is determined from the specifications for maximum pulling power (uphill power) required for the battery forklift.
(2) Point (c) is suitably determined in the mid speed region of the rotational speed range of the propulsion motor 4 (horizontal axis in FIG. 8), with an intersection point of the rating current of the propulsion motor 4 and the motor rating output curve (the curve shown by the chain line in FIG. 8) as a criterion. A condition here is that, with the point (c), the chopper duty is at 100%, that is to say the battery power source voltage is applied to the propulsion motor 4.
(3) Point (b) is a point on a straight line drawn parallel to the horizontal axis from the point (a), and is the point that gives the same value battery current flow in the low speed region of the rotational speed range of the propulsion motor 4, as the battery current at the point (c).
That is to say, in the low speed region, even in the case where the motor current (current command value) is large, it is possible to make the current supplied by the battery 1 have the same value. The reason is that the current flows continuously via a free wheel diode of a chopper circuit for inertia of the propulsion motor 4. Consequently, in this case, even if the duty is for example around 50%, a motor current of a current command value corresponding to the point (a) can be obtained.
The characteristic curve from (b) to (c) may be determined in accordance with the above-mentioned determination method for the point (b).
(4) Point (d) is a point on the rating output curve for the propulsion motor 4, and is the point corresponding to the limit speed of the propulsion motor 4, that is to say to the limit speed of the battery forklift.
Here the characteristic curve from (c) to (d) coincides with the rated output curve of the propulsion motor 4.
(5) Point (e) is a point corresponding to a pre-load of the battery forklift.
After determining the motor rotation speed/current command value function for the battery forklift by such a method, the motor rotation speed/current command value function corresponding to respective accelerator divisions is determined so as to obtain the desired torque and speed with respect to accelerator opening.
With the motor rotation speed/current command value function determined in this way, then of course a desired maximum torque can be obtained. Moreover, in going from the low speed region to the middle speed region, that is from point (b) to point (c), by keeping the duty of the chopper circuit as small as possible, then the battery consumption can be controlled.
Furthermore, while the characteristic curve from point (c) to point (d) is the rated output characteristic for the propulsion motor 4, the propulsion motor 4 is controlled to ensure a maximum speed, by reducing the current command value from point (d) to point (e)
Moreover, with the above described conventional battery forklift, since the voltage is controlled by duty controlling the propulsion motor 4, then the current value for the propulsion motor 4 is determined from the current which gives a balance at the before-mentioned duty ratio voltage corresponding to the steering angle, that is the angle subtended between the common axis of the two front wheels and the axis of the rear wheel. Consequently, at low speeds, the characteristic curves of the front wheel on the inside when turning (referred to hereunder as the inner wheel) and of the front wheel on the outside (referred to hereunder as the outer wheel) overlap, and hence both the inner and outer wheels have the same current value. Therefore, an undesirable influence is exerted on the feel during turning. Moreover, there is the problem of a reduction in turning efficiency attributable to friction etc. of the rear wheel.
The present invention takes into consideration the above situation with the object of providing a propulsion motor control apparatus for a battery vehicle, which can improve the feel during turning and the turning efficiency even at low speeds, and thus realize optimum turning performance.
In order to address the above problems, the propulsion motor control apparatus for a battery vehicle according to the present invention, being an apparatus which controls propulsion of a battery vehicle with a battery as a power source by individually controlling respective propulsion motors provided for two front wheels having a common axis, comprises; a steering angle detection device for detecting a steering angle subtended between the common axis of the two front wheels and an axis of a rear wheel, a ratio computing device for computing a ratio between a distance between an intersection point of the two axes and a first front wheel on a side further from the intersection point, and a distance between the intersection point and a second front wheel on a side closer to the intersection point, based on the steering angle, a function determining device for obtaining a motor rotation speed/current command value function for the second front wheel based on the motor rotation speed/current command value function for the first front wheel and the obtained ratio, and a control device for individually controlling the respective propulsion motors for the two front wheels based on the two functions.
With the propulsion motor control apparatus for a battery vehicle according to the present invention, since this comprises; the steering angle detection device for detecting the steering angle subtended between the common axis of the two front wheels, and the axis of the rear wheel, the ratio computing device for computing the ratio between the distance between the intersection point of the two axes and the first front wheel on the side further from the intersection point, and the distance between the intersection point and the second front wheel on the side closer to the intersection point, based on the steering angle, the function determining device for obtaining the motor rotation speed/current command value function for the second front wheel based on the motor rotation speed/current command value function for the first front wheel and the obtained ratio, and the control device for individually controlling the respective propulsion motors for the two front wheels based on the two functions, then by outputting current command values independent of each other respectively for the first front wheel and the second front wheel, corresponding to the steering angle, the respective propulsion motors for the two front wheels can be individually controlled. Consequently, the feel during turning and the turning efficiency at low speeds can be improved, and optimum turning performance can be realized.